Unless you're looking for some specific version a 3rd party package has been compiled against, it's called curl-devel in RHEL 5.
yum install curl-devel
# rpm -qpl curl-devel-7.15.5-17.el5_9.x86_64.rpm | grep libcurl
/usr/lib64/libcurl.a
/usr/lib64/libcurl.so
/usr/lib64/pkgconfig/libcurl.pc
/usr/share/man/man3/libcurl-easy.3.gz
/usr/share/man/man3/libcurl-errors.3.gz
/usr/share/man/man3/libcurl-multi.3.gz
/usr/share/man/man3/libcurl-share.3.gz
/usr/share/man/man3/libcurl-tutorial.3.gz
/usr/share/man/man3/libcurl.3.gz
This is straight out of the distribution's repository, no 3rd party repository necessary.